The present invention relates to an overdrive control apparatus for controlling an automatic gear change of an overdrive in accordance with a running speed and an engine load of a vehicle such as an automobile.
In a conventional automatic gear change control apparatus with a three-speed gear overdrive for an ordinary car in an automatic gear change control apparatus used in an automobile, comprising first (low), second, third and overdrive gears, the gear commanded is coupled with a gear on the engine side by means of a pneumatic coupling called a pneumatic torque converter in accordance with gear change points (line) which are preset according to a sensed running speed (vehicle speed) and a sensed engine load. Only for use with the overdrive range, a manual switch is installed on an instrument panel. So long as the overdrive switch is ON, the overdrive gear is effective in operation. On the other hand, when it is OFF, the overdrive gear is ineffective, while the first to third gears are effective. In gearing up from the low to second gear range, the second to third gear range or the third to overdrive gear range, or in gearing down from the second to first gear range, the third to second gear range or the overdrive to third gear range, a fixed zone, i.e. a hysteresis characteristic, is set up in the vehicle speed at the gear-up point and at the gear-down point, in order to prevent hunting.
The automatic gear change apparatus has no problem in particular in a level road drive because it has a change gear point to provide the best fuel consumption in the level road drive. On an ascending road with a larger gradient than a given value, the fuel consumption, drivability and acceleration are all better in the third gear range than in the overdrive range. Nevertheless, since that gear change point is set so as to improve the fuel consumption in the level road drive, no gear down shift from the overdrive to the third gear range takes place when the vehicle comes to an ascending road with such a gradient. Therefore, the vehicle runs in an ineffective drive condition, that is, with large fuel consumption, poor drivability and poor acceleration. Further in the overdrive range, the engine braking is only partially effective on the descending road. Therefore, the automatic gear change control device of the prior art is disadvantageous from the view point of safety in driving. Therefore, the manual overdrive switch must be operated every time the car comes to the ascending or descending road. The manual switching operation is troublesome for the driver. The troublesome manipulation taxes the limited mechanical ability of the drivers and therefore is undesirable from a viewpoint of the driving safety.